344 



7. INHIBITION IN MULTIENZYME SYSTEMS 



Another multienzyme system of common occurrence involves the transfor- 

 mation of an intermediate back to the substrate: 



E. E. 

 E, 



(7-29) 



Phosphatases and deacylases hydrolyzing phosphorylated or acylated in- 

 termediates respectively often are responsible for such systems. An espe- 



OOI 



100 



'3 



Fig. 7-14. Noncompetitive inhibition of Eg in a divergent cliain. The variation of 

 V2 with F3 i.s indicated. Inhibition of E3 will reduce F3 and d{C)/dt will increase, illus- 

 trating the stimulation of one branch by the inhibition of the other branch. Fj = 1, 

 Fj, = 1, K^^ 5 mM, K^ = 2 m3I, K^ = 10 mM, and (A) = 5 mM. 



cially important instance is the formation of glucose-6-phosphate catalyzed 

 by glucokinase and the hydrolysis of glucose-6-phosphate back to glucose 

 by a phosphatase. Actually the system formulated above is a type of di- 

 vergent chain and follows kinetics identical to those of the divergent chain 

 7-25, inasmuch as the individual rate expressions are the same. Inhibition 

 and activation of the enzymes involved may be of great importance in the 

 regulation of the metabolism as related to the needs of the tissue (Cahill 

 et al., 1959). Systems of this type involving phosporylation from ATP 

 can act as ATPases, since at each round of the cycle an ATP is split, as 

 pointed out by Krebs et al., (1958) for the 



2 phosphorylase b 



(1) 



phosphorylase a 



b ^IIZI^ phosphorylase a 



(2) 



